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March 2005
In this issue
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About us |
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Classes and seminars |
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Trick for this month |
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To contact us |
See our previous issues :
http://www.topcontrol.com/techinfo/news.php
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See our 2005 calendar, Classes & Seminars :
See at : http://www.topcontrol.com/training/tcalendar.php
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Classes & seminars
Mexico, March 15-17
SE-100-3 - Seminario en métodos de control modernos
http://www.topcontrol.com/training/printable.php?id=90&type=cours
Montreal, March 24-25
SE-120-2 - Seminar on performance assessment techniques using PlantTriage
http://www.topcontrol.com/training/printable.php?id=29&type=cours
New-York, March 27, Easter, March 29-31
SE-130-3 - Seminar on advanced tools in process control
http://www.topcontrol.com/training/printable.php?id=28&type=cours
Montreal, April 19-21
EX-501-3 - Detailed Loop Analysis with ExperTune
http://www.topcontrol.com/training/printable.php?id=13&type=cours
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Trick for this month : Kp was reduced and oscillations appeared!
It is well known that if one reduces proportional gain (Kp) in a PID controller:
- loop will be more stable,
- response time will be longer,
- oscillation will be reduced,
- overshoot will be reduced,
- robustness (process changes sensitivity) will be increased,
- etc.
Sometimes, it is not true!
If the PID controller has a parallel structure, reducing the
proportional gain will change the balance between proportional action
and Integral action. In most controllers (non parallel, Ideal/ISA,
Series) the output from the Proportional (Kp X Error) feeds other
modes (Integral and Derivative) therefore, reducing Kp will reduce Integral and Derivative at the same time. In a parallel structure, all
modes are fed by the error.
For example, both controllers below are equivalent:
| Parallel PID |  |
Ideal PID | |
| Kp = 0.25 | |
Kp = 0.25 | |
| Kl = 6 r.p.m. (Tl = 10s) | |
Kl = 24 r.p.m. (Tl = 2.5s) |
If parallel controller Kp is reduced to 0.05, then Integral part remains the same and equivalent Ideal controller has now an Integral part stronger; the loop could start to oscillate. These oscillations will have a long period being induced by Integral; at first glance, they could be undetected.
| Parallel PID | |
Ideal PID | |
| Kp = 0.05 | |
Kp = 0.05 | |
| Kl = 6 r.p.m. (Tl = 10s) | |
Kl = 120 r.p.m. (Tl = 0.5s) |
Parallel controllers are popular in PLCs and in some DCSs. Flow loops cycling at long (minutes) period are frequent when parallel controllers are used; numbers seem acceptable but in fact sometimes they are not! Stay alert!
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Comments, suggestions and requests are welcomed. Please e-mail us at: info@topcontrol.com
You are welcomed to visit our web site :http://www.topcontrol.com
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49 Bel-Air Street, Lévis, QC
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1020 James Drive, Suite A
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